1. Field of the Invention
The present invention relates to the field of timing pattern generation for self-servo-writing magnetic drives.
2. Description of Related Art
High track densities in rotating media mass storage devices are becoming possible with newer drive technologies. These new technologies include voice-coil and other types of servo positioners as well as the ability to read and write narrower tracks by using, for example, magneto-resistive (MR) head technology. Higher track densities increase the accuracy requirements of servowriting methods for embedded servo systems needed to position the head.
Conventional disk drive manufacturing techniques, for example, include writing servo-tracks on the media of a head disk assembly (HDA) with a specialized servo-writer instrument. Laser positioning feedback is used in such instruments to read the actual physical position of a recording head used to write the servo-tracks. Unfortunately, it is becoming more and more difficult for such servo-writers to invade the internal environment of a HDA for servo-writing because the HDAs themselves depend on their covers and castings being in place for proper operation. Also, some HDAs are very small, less than 2 inches square At such levels of microminiaturization, traditional servo-writing methods are inadequate.
Conventional servo-patterns typically comprise short bursts of a constant frequency signal, very precisely located offset on either side from a data track's center line. The bursts are written in a sector header area, and can be used to find the center line of a track. Staying on center is required during both reading and writing. Since there can be 100 or more sectors per track, that same number of servo data areas must be dispersed around a data track.
Further, the servo-data is generally dispersed around the data track by writing short bursts in each of the hundred or so sector header areas of the data track. Such data bursts can be used by the embedded servo mechanism to find the center line of the data track. This allows the head to follow the track center line around the disk even when the track is out of round (e.g., due to spindle wobble, disk slip, and/or thermal expansion). As the capacity of disk drives increases track density is likewise increased, the servo-data must be more accurately located on the disk.
Servo-data is conventionally written by dedicated, external servo-writing equipment, and typically involves the use of granite blocks to support the disk drive and quiet outside vibration effects. An auxiliary clock head is inserted onto the surface of the recording disk and is used to write a reference timing pattern. An external head/arm positioner with a very accurate lead screw and a laser displacement measurement device for positional feedback is used to precisely determine transducer location. This precise transducer location is the basis for track placement and track-to-track spacing. The servo writer requires a clean room environment, as the disk and heads are exposed to the environment to allow the access of the external head and actuator.
A conventional servo-data pattern on a disk comprises circular data tracks that are broken into sectors. Each sector typically has a sector header area followed by a data area. Each sector header area includes sector header information followed by a servo-data area that provides radial position information. The sector header information includes a servo-identification (SID) field and a gray code field that must be precisely aligned from track to track to prevent destructive interference in the magnetic pattern. Such interference can reduce the amplitude of the signal and cause data errors.
During conventional drive manufacturing, the disk drive is typically mounted in a mastering station that is known as a servo-writer. The servo-writer has sensors that are positioned outside of the disk drive to locate the radial and circumferential position of at least one of the drive's internal heads. Using information from the sensors, the servo-writer causes the head to write a pattern, typically magnetic information, (i.e., servo-data) onto the disk. As explained above, the servo-pattern becomes the master reference used by the disk drive during normal operation to locate the tracks and sectors for data storage. When such a station is used to perform the servo-writing, manufacturing expenses increase because each disk drive must be mounted in the servo-writer. Additionally, the mechanical boundary conditions of the disk are altered because the external sensors must have access to the actuator and the disk spindle motor. Thus, mechanical clamping and disassembly of the drive may also be required.
According to another conventional servo-writing process, a master clock track is first written on the disk by a separate head to serve as a timing reference for the entire servo-track writing operation. After writing the master clock track, servo-data bursts are written over the entire surface of the disk by first moving the arm to the outer crash stop and then radially moving the arm a distance that is less than a data track width using an external radial positioning system for each revolution of the disk.
Such conventional servo-writing procedures require the use of an external timing sensor in order to write the timing patterns that are used to determine the circumferential head position. Because external sensors are needed, the servo-writing must be performed in a clean room environment. Additionally, an external clock source and auxiliary clock heads are required to write the timing information.
To overcome such problems, self-servo-writing timing generation processes have recently been developed. These processes allow accurately aligned servo-data tracks to be written sequentially at each servo data radius without using any mechanical, magnetic, or optical positioning systems to control the circumferential positioning of the servo data. Further, the need for auxiliary clock heads to write a reference timing pattern on the disk is eliminated.
According to one method, first timing marks are written at a first radial position of the storage medium. Time intervals between selected pairs of the first timing marks are measured. The head is moved to a second radial position. Next, additional timing marks are written by recording the time of passage of every other timing mark (say the odd numbered ones) during revolutions of the disk and then writing the intervening time marks (the even numbered ones) at calculated delays thereafter. The time intervals between the newly written (even) marks are estimated to be the difference in times of passage of the adjacent timing (odd) marks plus the difference in the delay before writing the new timing marks. Then the head is moved to a second radial position. Next, additional timing marks are written by recording the time of passage of every other timing marks at the circumferential positions just written (here the even numbered ones) during revolutions of the disk and then writing the intervening time marks (the odd numbered ones) at calculated delays thereafter.
The time intervals between the newly written (odd) marks are estimated to be the difference in times of passage of the adjacent timing (even) marks plus the difference in the delay before writing the new timing marks. In the preferred method, servo data is written on one or more disk surfaces in the intervals between the timing marks. In a preferred method, the steps of measuring, moving, and writing other timing marks are repeated until the servo-pattern is written on an entire surface of the storage medium.
While such self-servo-writing processes are sufficient when the servo-data tracks are to be written using overlapping read and write heads (i.e., where a track can be written and read without changing head position), disk drives with non-overlapping read and write elements are now being produced.
More specifically, as read and write element dimensions have been decreased to increase storage density, the widths over which reading and writing occur have decreased more rapidly than the distance between the read and write elements themselves. As a result, when using a head with such elements on a rotary actuator, the read element of the head can no longer overlap the area written by the write element of the head at all radial positions. When the above self-servo-writing processes are used for drives in which the read and write elements do not overlap, accurate circumferential alignment of the servo-data tracks is not maintained and there is a lack of stability against the growth of random errors in the pattern generation process.
According to another method, first timing marks are written at a first radial position of the storage medium during revolutions of the disk. Then the head is moved to a second radial position. Time intervals between selected pairs of the first timing marks are measured during revolutions of the disk. Next, additional timing marks are written by recording the time of passage of every other timing mark (say the odd numbered ones) during revolutions of the disk and then writing the intervening time marks (the even numbered ones) at calculated delays thereafter. Then the head is moved to a second radial position. Time intervals between selected pairs of the first timing marks are measured during revolutions of the disk. Next, additional timing marks are written by recording the time of passage of every other timing marks at the circumferential positions just written (here the even numbered ones) during revolutions of the disk and then writing the intervening time marks (the odd numbered ones) at calculated delays thereafter. In a preferred method, the steps of moving, measuring, and writing other timing marks are repeated until the servo-pattern is written on an entire surface of the storage medium.
Commonly owned U.S. Pat. application Ser. No. 09/592,740, filed Jun. 13, 2000 and entitled “Method for Self-Servo Writing Timing Propagation” is hereby incorporated by reference in its entirety. This U.S. Patent Application (heretofore referred to as the '740 application) describes a self-servo-writing process. The placement of new timing marks had previously occurred at least every other revolution to allow reading of, and measuring all the time intervals between, existing timing marks at least during a revolution before writing a subsequent new timing mark. In addition, with all of these processes only half of the timing mark locations are written at each radial position. This, unfortunately, can result in odd-even sector asymmetry, reduced signal strength at the timing mark, and increases the overall time between measurements during which the motor speed can significantly vary possibly introducing additional timing errors into measurements of timing mark locations.
Further, another type of reading and writing apparatus uses an offset head. In an “offset” head, the read and write elements are physically separated in the radial direction. An offset head includes a recording head, otherwise known as a write head, and a magnetic detection head, otherwise known as a read head. A prior art offset writing process requires that additional timing measurements be made to maintain process stability which adds to process time.
The invention of the '740 application overcame problems with the prior art by detecting both the passage of the timing marks and writing extensions to timing marks at substantially the same circumferential positions. This is feasible even if a disk drive or similar system is unable to write and read at the same time, if the read head is a separate element that encounters points on a disk surface slightly before the write head as the disk rotates so that the detection of an existing timing mark can take place before the writing of a timing mark in the same circumferential location. After the read operation occurs this delay allows the subsequent write operation at substantially the same tangential location on the same revolution. Using this process which records the passage of every timing mark and then writes extensions to all of those timing marks at substantially the same circumferential positions improves the accuracy of timing mark placement and produces a commensurate improvement in the placement of the concomitantly written servo data.
Another invention allows this last higher accuracy method where all timing mark locations are both detected and written during the same disk revolution to be executed without performing any other measurement steps except those made during that disk revolution. This reduces the overall process time. This invention is disclosed in co-pending U.S. Pat. application Ser. No. 09/426,435, filed Oct. 25, 1999 and entitled “Storage of Timing Information for Self-Servo Writing Timing Pattern Generation When Read and Write Heads are Non-Overlapping”, which defines a location array which stores the estimated intervals between newly written timing marks calculated from the measured timing mark intervals of the timing marks that are detected by the read head and the delays used to write new timing marks in a data array. This U.S. Patent Application (heretofore referred to as the '435 application) is commonly assigned herewith to International Business Machines, and is hereby incorporated by reference in its entirety. At each new writing step the stored estimated interval data for the timing marks currently passing under the read head is retrieved and used to predict the correct delays for writing. This means it is not necessary to measure the time intervals between the timing marks passing under the read head before the disk revolution that new timing marks are written.
The combination of the inventions of the '740 application and the '435 application allows for both 1) high accuracy, since according to the invention of the '740 application every timing mark is both read and written on each step in the same revolution (rotation) of a disk in a drive, and 2) high process speed, since according to the invention of the '435 application it is not necessary to take the time to measure the time intervals between the timing marks passing under the read head before the disk revolution that new timing marks are written. However the invention of the '740 application requires particularly, heads that can read and then write at the same circumferential location on the same revolution via the presence of a significant delay that allows the subsequent write operation at substantially the same tangential location on the same revolution. The delay requirement also limits the duration (circumferential extent) of a timing mark that can be written since the reading of the timing mark must be completed before the write element reaches the leading edge of the set of extensions to that timing mark. This constraint limits the types of recording head and timing mark patterns that can be used with the higher accuracy method where every timing mark is both read and written in the same revolution.
Accordingly, there exists a need to overcome the problems with the prior art as discussed above, and particularly for a method to more effectively write timing marks on rotatable storage media.